The use of free piston arrangements in various types of engines particularly Stirling type engines is well known. The basic design involves the use of a displacer and a power piston. Typically, the displacer is positioned in a cylinder defining expansion and compression chambers. Coupled between these chambers may be a heat exchanger through which gas passes. For example, in a refrigeration setting the displacer on which a mechanical reciprocal movement is imparted, reciprocates between upper and lower points. At the lower point compressed gas is admitted into the compression chamber which is then compressed upon movement of the displacer. The gas then passes through the heat exchanger where the gas exchanges heat with it and into the expansion space where it undergoes adiabatic expansion which decreases its temperature. When the displacer moves down, the gas in the expansion chamber is forced through the heat exchanger, removing heat therefrom. The cycle then repeats itself continually to produce a heat sink.
Generally speaking the purpose of the displacer is to transfer engine working gas from hot volume to the cold volume and back during engine operation. The purpose of the power piston is alternately to compress and expand engine working gas, and transfer the resulting mechanical power to a suitable load. Both the displacer and the power piston are connected to gas springs. The purpose of the gas springs is to provide reactive power flow required alternately to accelerate and decelerate these reciprocating elements. Pressure waves in the gas spring are isolated from the engine pressure wave by close clearance seals. These pressure waves are not however in phase with each other. Therefore, a time varying pressure gradient exists across the reciprocating elements which are generally supported on gas bearings.
A time varying pressure gradient across the gas bearings can result in a large oscillation of the bearing journal. Time varying pressure gradient across the gas bearing of a free-piston machine and close clearance seals can also impose significant dynamic forces and moments on the reciprocating elements. Pressure gradient in the clearance seals affects engine performance due to power loss associated with the seal leakage. It also affects the dynamic behavior of the reciprocating elements by imposing time varying force and moment.
For a given pressure differential and seal radial clearance, leakage power loss is inversely proportional to the seal length and the dynamic load is directly proportional to the square of the seal length. Therefore, from engine performance considerations the seal length needs to be large, and from the bearing consideration (which carries the dynamic loads) the seal length needs to be short.
Accordingly, there exists a need to reduce the dynamic loads on the elements without impacting the engines performance.